/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.expression.spel.ast;

import java.lang.reflect.Modifier;

import org.springframework.asm.MethodVisitor;
import org.springframework.expression.EvaluationContext;
import org.springframework.expression.TypedValue;
import org.springframework.expression.spel.CodeFlow;
import org.springframework.expression.spel.ExpressionState;
import org.springframework.expression.spel.SpelEvaluationException;
import org.springframework.lang.Nullable;

/**
 * Represents a variable reference, eg. #someVar. Note this is different to a *local*
 * variable like $someVar
 *
 * @author Andy Clement
 * @since 3.0
 */
public class VariableReference extends SpelNodeImpl {

    // Well known variables:
    private static final String THIS = "this";  // currently active context object

    private static final String ROOT = "root";  // root context object


    private final String name;


    public VariableReference(String variableName, int pos) {
        super(pos);
        this.name = variableName;
    }


    @Override
    public ValueRef getValueRef(ExpressionState state) throws SpelEvaluationException {
        if (this.name.equals(THIS)) {
            return new ValueRef.TypedValueHolderValueRef(state.getActiveContextObject(), this);
        }
        if (this.name.equals(ROOT)) {
            return new ValueRef.TypedValueHolderValueRef(state.getRootContextObject(), this);
        }
        TypedValue result = state.lookupVariable(this.name);
        // a null value will mean either the value was null or the variable was not found
        return new VariableRef(this.name, result, state.getEvaluationContext());
    }

    @Override
    public TypedValue getValueInternal(ExpressionState state) throws SpelEvaluationException {
        if (this.name.equals(THIS)) {
            return state.getActiveContextObject();
        }
        if (this.name.equals(ROOT)) {
            TypedValue result = state.getRootContextObject();
            this.exitTypeDescriptor = CodeFlow.toDescriptorFromObject(result.getValue());
            return result;
        }
        TypedValue result = state.lookupVariable(this.name);
        Object value = result.getValue();
        if (value == null || !Modifier.isPublic(value.getClass().getModifiers())) {
            // If the type is not public then when generateCode produces a checkcast to it
            // then an IllegalAccessError will occur.
            // If resorting to Object isn't sufficient, the hierarchy could be traversed for
            // the first public type.
            this.exitTypeDescriptor = "Ljava/lang/Object";
        }
        else {
            this.exitTypeDescriptor = CodeFlow.toDescriptorFromObject(value);
        }
        // a null value will mean either the value was null or the variable was not found
        return result;
    }

    @Override
    public void setValue(ExpressionState state, @Nullable Object value) throws SpelEvaluationException {
        state.setVariable(this.name, value);
    }

    @Override
    public String toStringAST() {
        return "#" + this.name;
    }

    @Override
    public boolean isWritable(ExpressionState expressionState) throws SpelEvaluationException {
        return !(this.name.equals(THIS) || this.name.equals(ROOT));
    }

    @Override
    public boolean isCompilable() {
        return (this.exitTypeDescriptor != null);
    }

    @Override
    public void generateCode(MethodVisitor mv, CodeFlow cf) {
        if (this.name.equals(ROOT)) {
            mv.visitVarInsn(ALOAD, 1);
        }
        else {
            mv.visitVarInsn(ALOAD, 2);
            mv.visitLdcInsn(this.name);
            mv.visitMethodInsn(INVOKEINTERFACE, "org/springframework/expression/EvaluationContext", "lookupVariable", "(Ljava/lang/String;)Ljava/lang/Object;", true);
        }
        CodeFlow.insertCheckCast(mv, this.exitTypeDescriptor);
        cf.pushDescriptor(this.exitTypeDescriptor);
    }


    private static class VariableRef implements ValueRef {

        private final String name;

        private final TypedValue value;

        private final EvaluationContext evaluationContext;

        public VariableRef(String name, TypedValue value, EvaluationContext evaluationContext) {
            this.name = name;
            this.value = value;
            this.evaluationContext = evaluationContext;
        }

        @Override
        public TypedValue getValue() {
            return this.value;
        }

        @Override
        public void setValue(@Nullable Object newValue) {
            this.evaluationContext.setVariable(this.name, newValue);
        }

        @Override
        public boolean isWritable() {
            return true;
        }
    }

}
